1. Technical Field
The present invention relates to a method for producing a semiconductor device by a transfer technique.
2. Related Art
In recent years, there have been advances in the development of thin, lightweight, freely foldable flexible devices. For example, flexible displays typified by electronic paper have lightness, shock absorbing properties, and flexibility such that they conform to hands and thus can be electronic devices that play a role in a ubiquitous society. As such electronic devices, a device including a thin-film element, such as a thin-film transistor, formed on a flexible plastic substrate using a transfer technique is known, as disclosed in, for example, JP-A-2004-140383.
In a transfer process according to the related art, a separation layer is formed on a glass substrate serving as a transfer-source substrate. A thin-film element, which is a transfer layer, is formed thereon by a normal step. The transfer-source substrate is bonded to a plastic substrate, which is a transfer-target substrate, with an adhesive. The adhesion between the separation layer and the transfer-source substrate is reduced using light, heat, or the like. The transfer layer is transferred to the transfer-target substrate. The transfer of the transfer layer to the plastic substrate may be performed through two or more transfer steps. For example, in JP-A-2004-327836, the transfer layer is transferred from the transfer-source substrate to a glass substrate, which is a first transfer-target substrate. Then the transfer layer is transferred to a plastic substrate, which is a second transfer-target substrate. Thereby, the stacking relationship of the transfer layer provided on the transfer-source substrate can be reproduced on the transfer-target substrate.
In such electronic devices, the distance between the transfer-target substrate and the transfer layer is defined by the thickness of the adhesive. Thus, the thickness of the adhesive is required to be uniform in the entire transfer region. For example, a liquid adhesive is used to bond (superpose) the transfer-source substrate to the transfer-target substrate. In this case, although both substrates are planned to be superposed parallel to each other, the substrates may be tilted due to nonuniform weight distribution. The transfer layer held by the adhesive cured while the substrates are tilted is not arranged so as to have a uniform height from the plane of the transfer-target substrate, thereby possibly causing various problems in mounting. In particular, these problems become pronounced when the substrates are large.
To overcome these problems, JP-A-2004-327836 discloses a process of forming a surrounding member having a uniform thickness around a region to which an adhesive is applied and charging the adhesive thereinto. The surrounding member is composed of a material capable of uniformly forming the surrounding member and having sufficient strength such that the spacing can be temporarily maintained. JP-A-2004-327836 discloses the surrounding member formed of a nitride film or an oxide film having a uniform thickness formed by known CVD or sputtering or the surrounding member formed by patterning a resin film using photolithography.